Various linear operations such as document scanning and printing utilize the synchronization of images relative to a moving web of material. For example, a hand-held scanner can be synchronized to a document being scanned for ascertaining the relative movement between the scanner and the document for accurately picking up an image from the document. Similarly, the movement of paper through a printer can be monitored to more accurately align the transfer of print to the paper. Further, the movement of assembly components along a conveyor belt can be monitored to help ensure timely delivery of components to assembly stations in a device production facility, such as a car manufacturing plant. In each of these examples, the movement of images are tracked along a moving web or plane.
Navigation is a term that has been applied to tracking, or monitoring, the position of images along a moving web. At least a reference image and a comparison image are acquired at different times, and a moving web is tracked or monitored by navigating between the two images. When light beams are used to assist with the tracking and/or monitoring, the process has been called optical navigation, whether or not the light beams are visible to the human eye. One system applying optical navigation to track the movement of a sheet of paper is disclosed in commonly assigned U.S. Pat. No. 5,149,980 to Ertel et al.
While the moving web in the above example is a moving sheet of paper, optical navigation can be also be applied to images being transported along a moving web. One system for measuring the velocity, displacement, and strain along a moving web in an application of optical navigation is disclosed in U.S. Pat. No. 6,118,132 to Tullis. Regardless of the web in use, optical navigation includes one or more image acquisition devices, such as an optical sensor, including a camera, for capturing an image along the web. By subsequent acquiring of images along the web and successively comparing the images to each other, the progress of the web and/or images or objects on the web can be monitored. U.S. Pat. No. 6,195,475 to Raymond G. Beausoleil, Jr. discloses a system for navigating among images captured by a scanning device. Based on the progression of the objects or images along the web, a process, such as document printing or scanning, can be monitored and/or controlled.
Navigation from image to image along a moving web can be complicated by the blurring of the images as captured by an image acquisition device. The blurring is a function of the shutter speed, or exposure time, and the web velocity as an image moves through the field of vision of the image acquisition device while the aperture is open. Shutter speed is understood to be the measurement of time, such as 1/1000 of a second, that the shutter of an image acquisition device is open for capturing an image. The blurring renders the image less distinct and precise and renders the process of matching between images more problematic; and, accordingly, can introduce errors into the navigation process. The effect of the blurring can be reduced by a slower web velocity and/or by a faster shutter speed, but such choices can reduce throughput or increase system cost. The blurring of the images is further complicated when the web moves at an inconsistent velocity, such as when the web accelerates or decelerates between the times the images are acquired. Under conditions of inconsistent velocity, navigation from image to image becomes more difficult, and navigation errors can increase because of the differing degrees of blurring of the images and the consequential difficulty in matching the dissimilar images to each other. What is needed is a system and method for performing optical navigation.